Answer:
mountains
Step-by-step explanation:
Around 80% Of Greece's territory is largely mountainous, and Greek communities in the past decided that those mountains could serve as natural borders to easily distinguish one communities from another.
These mountains benefited the Greek in terms of military positioning. The mountains provide a natural fortress that made it really hard for outsiders to attack Greek communities.
On the downside, these mountains made it really hard for Greek communities to interact/socialize with one another. Since it's harder to establish a close relationship with one another, This improve the risk of conflict between those communities.